1. Field of the Invention
This invention relates generally to a stand-alone fuel processor for a hydrogen fuel cell engine and, more particularly, to a stand-alone fuel processor for a hydrogen fuel cell engine in a vehicle that employs a rapid-cycle pressure swing adsorber device for removing carbon monoxide (CO) and other by-products from the reformate gas to produce high purity hydrogen.
2. Discussion of the Related Art
Hydrogen is a very attractive source of fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles with internal combustion engines. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode having, an electrolyte therebetween. The anode receives a hydrogen gas and the cathode receives oxygen. The hydrogen gas is ionized in the anode to generate free hydrogen ions and electrons. The hydrogen ions pass through the electrolyte to the cathode. The hydrogen ions react with the oxygen and, the electrons in the cathode to generate water as a by-product. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work, before being sent to the cathode. The work acts to operate the vehicle. Many fuels cells are combined in a stack to generate the desired power.
Proton exchange membrane (PEM) type fuel cells are a popular fuel cell for vehicles. In a PEM fuel cell, hydrogen (H2) is the anode reactant, i.e., fuel, and oxygen is the cathode reactant, i.e., oxidant. The cathode reactant can be either pure oxygen (O2) or air (a mixture of O2 and N2). The PEM fuel cell typically includes a solid polymer electrolyte, typically made from ion exchange resins, such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles and mixed with a proton conductive resin. The catalytic particles are typically precious metal particles, and thus are costly. These membrane electrode assemblies are relatively expensive to manufacturer and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst fouling constituents, such as CO.
In vehicle fuel cell applications, it is desirable to use a liquid fuel, such as alcohols (methanol or ethanol), hydrocarbons (gasoline), and/or mixtures thereof, such as blends of ethanol/methanol and gasoline, as a source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store on the vehicle. Further, there is a nationwide infrastructure for supplying liquid fuels. Gaseous hydrocarbons, such as methane, propane, natural gas, LPG, etc., are also suitable fuels for both vehicle and non-vehicle fuel cell applications.
Hydrocarbon-based fuels must be disassociated to release the hydrogen therefrom for fueling the cell. The disassociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors where the fuel reacts with steam, and sometimes air, to generate a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in a steam-methanol reformation process, methanol and water are reacted to generate hydrogen and carbon dioxide. However, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor that contains two sections. One section is primarily a partial oxidation reactor (POX) and the other section is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water.
The known fuel processors also typically include downstream reactors, such as a water/gas shift (WGS) reactor and a preferential oxidation (PROX) reactor. The PROX reactor is used to remove carbon monoxide in the reformate gas because carbon monoxide contaminates the catalytic particles in the PEM fuel cell. The PROX reactor selectively oxidizes carbon monoxide in the presence of hydrogen to produce carbon dioxide (CO2) using oxygen from air as an oxidant. However, the use of a PROX reactor in a fuel processor adversely effects processor performance. For example, control of the air feed is important to selectively oxidize CO to CO2. Also, the PROX reactor is not 100% selective, and thus results in consumption of hydrogen. Thus, the hydrogen that would normally be available to provide power is consumed by the PROX reactor. Further, the heat generated from the PROX reactor is at a low temperature, resulting in excess low-grade heat. Also, typical catalysts used in a PROX reactor contain precious metals, such as platinum or iridium, which are very expensive.
The gasoline fuel processor technology to date requires large start-up durations, large mass and large volume. The start-up time for such a system is limited by the time until the combination of the WGS reactor and the PROX reactor can reduce CO to an acceptable level to supply stack grade hydrogen. The start-up duration is determined by the mass of the catalyst system used for start-up, and the energy needed to get the catalyst system up to temperature. Another limitation of the current technology is how to utilize all of the low-grade heat that the system will generate. Any excess heat loss causes a reduction in fuel processor efficiency.
The H2 generated in a fuel processor using a PROX reactor for CO clean-up typically contains less than 50% H2, where the balance of the hydrogen-rich reformate consists primarily of carbon dioxide, nitrogen and water. Thus, the reformate is not suitable for compression and storage because much energy would be wasted in compressing the non-H2 components in the reformate gas. Also, valuable storage space would be wasted to contain the non-H2 components.
Some techniques do exist in the art for generating nearly pure H2 in non-automotive fuel processing systems. One technique of generating pure H2 in a fuel processing system involves the use of H2 permeable membranes. These membranes selectively allow the H2 to pass through and prevent the other by-product constituents in the reformate gas from permeating through. Typical membranes for these applications contain palladium, which is very expensive. Also, these membranes only operate at relatively high temperatures (250-550° C.), and thus, it takes a long time after the low temperature start-up for a fuel processing system containing H2 permeable membranes to be able to generate H2. Additionally, these membranes operate at very high pressures (>5 bar), which leads to exorbitant compressor loads and inefficient systems.
Another technique of generating essentially pure H2 from a fuel processor using a hydrocarbon fuel employs adsorption. For example, a pressure swing adsorber (PSA) could be used to generate pure hydrogen from the reformate gas of a WGS reactor. Such a process is described in commonly owned U.S. patent application Ser. No. 09/780,184, published Aug. 15, 2002 as publication No. US 2002/0110504 A1, and herein incorporated by reference. In the fuel cell system disclosed in the '184 application, the PSA is integrated with the fuel cell stack. The PSA uses the anode off-gas from the fuel cell as a purge stream within the PSA or uses the cathode off-gas from the fuel cell to combust the low-pressure exhaust gas from the PSA. Additionally, both the anode and cathode off-gas can be used. Such a system could not be used as a stand-alone H2 generator, where the H2 gas is stored for subsequent use in a fuel cell engine. Similar processes for H2 generation for fuel cells using a PSA are described in U.S. patent application Ser. No. 09/808,175, published Jan. 10, 2002, Publication No. US 2002/0004157 A1, and International Patent application, No. WO 00/16425, published Mar. 23, 2000.